Subaperture Array Research Articles

A Multiplexed 32 × 32 2D Matrix Array Transducer for Flexible Sub-Aperture Volumetric Ultrasound Imaging.

A fully-sampled two-dimensional (2D) matrix array ultrasonic transducer is essential for fast and accurate three-dimensional (3D) volumetric ultrasound imaging. However, these arrays, usually consisting of thousands of elements, not only face challenges of poor performance and complex wiring due to high-density elements and small element sizes but also put high requirements for electronic systems. Current commercially available fully-sampled matrix arrays, dividing the aperture into four fixed sub-apertures to reduce system channels through multiplexing are widely used. However, the fixed sub-aperture configuration limits imaging flexibility and the gaps between sub-apertures lead to reduced imaging quality. In this study, we propose a high-performance multiplexed matrix array by the design of 1-3 piezocomposite and gapless sub-aperture configuration, as well as optimized matching layer materials. Furthermore, we introduce a sub-aperture volumetric imaging method based on the designed matrix array, enabling high-quality and flexible 3D ultrasound imaging with a low-cost 256-channel system. The influence of imaging parameters, including the number of sub-apertures and steering angle on imaging quality was investigated by simulation, in vitro and in vivo imaging experiments. The fabricated matrix array has a center frequency of 3.4 MHz and a -6 dB bandwidth of above 70%. The proposed sub-aperture volumetric imaging method demonstrated a 10% improvement in spatial resolution, a 19% increase in signal-to-noise ratio, and a 57.7% increase in contrast-to-noise ratio compared with the fixed sub-aperture array imaging method. This study provides a new strategy for high-quality volumetric ultrasound imaging with a low-cost system.

Even sampling photonic-integrated interferometric array for synthetic aperture imaging.

To improve the effectiveness of spatial spectrum sampling for the photonic-integrated interferometric imaging, an array forming scheme is proposed with evenly distributed interferometric baselines, which is referred to as the even sampling photonic-integrated interferometric array (ESPIA). The subaperture array of ESPIA is configured as equi-spaced concentric rings. The subaperture beams are coupled and transmitted to the photonic integrated circuit through fiber optic channels and paired into baselines by the interferometric beam combination. The characteristics of ESPIA are analyzed with the discrete modulation transfer function (D-MTF) and multi-resolution mutual information (MR-MI). The simulation results show that it can realize the even sampling coverage of spatial spectrum effectively. With the same scale of synthetic aperture and subaperture array, it can also improve the capabilities of information acquisition for the interferometric array.

Robust depth estimation on real-world light field images using Gaussian belief propagation

Depth estimation on light field images has been well researched on synthetic datasets. However, existing methods often fail on real-world light field images because of the real-world noises. Due to the lack of real-world light field image datasets for depth estimation, the learning-based methods, e.g., convolutional neural networks, are not capable of performing well on real-world light field images with only synthetic datasets available. In this paper, we adopt the optical flow estimation method to estimate depth from light field images since (i) the existing optical flow methods have shown robustness with significant real-world noises; (ii) the adopted optical flow estimation method does not require any training data, thus not limited to the synthetic datasets. The depth can be solved via the approach of optical flow estimation because (i) a sub-aperture image array can be converted from a light field image; (ii) the optical flow between every two adjacent sub-aperture images of the image array is exactly the disparity between the two images. Furthermore, since a sub-aperture array contains numerous adjacent image pairs, numerous optical flow maps can be generated from the sub-aperture array. We show the depth can be well refined from these numerous optical flow maps through modeling these maps in a single graphical model (a pairwise Gaussian MRF to be precise) and inferencing this graphical model by Gaussian belief propagation (GaBP). Experiments show that i) the proposed method achieves better depth estimation than the state-of-the-art methods on real-world light field images; ii) the proposed method can estimate the depth of objects at a far distance (3.5–9.5 m) much more accurately than the state-of-the-art convolutional neural networks.

Use of graphics processors to cardinally acceleration the calculation procedure of large subaperture active phased antenna arrays radiation pattern

For traditional APAA with a rectangular and triangular grid of nodes, in which partial emitters are installed, simple analytical formulas are known that express the dependence of the array radiation pattern (ARP) on the angular coordinates. If the grid of nodes is irregular, or in a regular grid there are failures of individual digits in the control elements (phase shifters, attenuators, delay lines) or in power amplifiers, then these formulas are not applicable. Therefore, it is necessary to address to general relations of additive summation of the specific contribution to the ARP of the field generated by each emitter separately. In large arrays, their number can range from several to many tens or even hundreds of thousands. Further, if it is necessary to synthesize a given type of array RP, it is necessary to form special types of the amplitude-phase distribution of the signal on the emitters. The procedures for synthesizing such distributions require hundreds or even thousands of repeated calculations of the array RP, which may require several million angular points to analyze in the upper hemisphere of radiation. The article shows that using parallel computing with the help of programming technology on graphics processors (GPU), produced by NVIDIA and CUDA technology in a system designed to quickly create high-performance codes in the extended C language, C++ is able to speed up the calculations of the large antenna ARP by 2…3 orders of magnitude, even on standard office computers. When using modern CPUs (AMD Ryzen Threadripper PRO 3995 WX) and GPU (NVIDIA GeForce 3090), the acceleration in calculations can additionally increase by hundreds of times. At such computing speeds, even the problems of antenna synthesis and statistical analysis of the characteristics of the array RP will be solved almost in real time. An example of a statistical estimation of the ARP parameters of a large subaperture array with 1280 emitters, used in the COSMOSkyMed system, when random failures of the inverse type occur in the phase shifter digits. The use of graphics processors allows us to solve problems such as the synthesis of an array RP having a prescribed shape, and the statistical analysis of the characteristics of the array in the event of various failure options in its elements, as well as mechanical distortions of the aperture shape, in almost real time.

Verification of resolution on self-focusing effect of polarization-inverted transmitter by up-chirp signal for subaperture array

In this study, we experimentally proved the relationship between the number of subapertures and the self-focusing effect of a polarization-inverted transmitter by up-chirp signal driving. First, experiments on the focusing effect by aperture division were carried out using a prototype polarization-inverted transmitter. As a result, the beam width was improved by dividing the aperture. By increasing the number of aperture divisions, the beam width was reduced and the distance resolution was improved proportionally to the number of divisions. Finally, the beam width at the focal point was measured by changing the frequency component of the up-chirp signal driving the transmitter. Dependent on the focal point, the beam width did not change significantly. As a result, a focal point could be formed within the near field owing to the self-convergence phenomenon of the polarization-inverted transmitter and the convergence effect caused by aperture division.

Optimization of sparse subaperture array model for stitching detection of plane wavefront

The sparse subaperture stitching, the accuracy of which is closely related to the arrangement, number and size of subapertures, is one of the main methods of quality testing for large aperture optical systems. A mathematical model was established to deduce the relation curve between the subaperture number k and fill factor M when the value of k ranges from one to infinity. As a result, the optimal arrangement layout, consisting of seven sparse subapertures, was obtained for the detection systems below 1.5 m. Autocollimation interference detection of Φ200 mm validated the rationality of the arrangement.

Digital phase correction of a partially coherent sparse aperture system.

A digital piston phase correction method is presented for partially coherent synthesis with active or passive illumination. An anamorphic pupil relay causes a linear shift of a two subaperture array between the entrance and exit pupils. This shift separates the subapertures' cross- and auto-correlations while retaining their common spatial frequency information. Digital analysis of these common frequencies finds the separation distance and piston phase error of the cross-correlations and enables lossless correction of phase error. Corrected images are diffraction limited. Partial coherence affects the contrast of each spatial frequency, causing optical path difference tolerances to relax as system bandwidth decreases.

Error analysis of subaperture processing in 1-D ultrasound arrays.

To simplify the medical ultrasound system and reduce the cost, several techniques have been proposed to reduce the interconnections between the ultrasound probe and the back-end console. Among them, subaperture processing (SAP) is the most straightforward approach and is widely used in commercial products. This paper reviews the most important error sources of SAP, such as static focusing, delay quantization, linear delay profile, and coarse apodization, and the impacts introduced by these errors are shown. We propose to use main lobe coherence loss as a simple classification of the quality of the beam profile for a given design. This figure-ofmerit (FoM) is evaluated by simulations with a 1-D ultrasound subaperture array setup. The analytical expressions and the coherence loss can work as a quick guideline in subaperture design by equalizing the merit degradations from different error sources, as well as minimizing the average or maximum loss over ranges. For the evaluated 1-D array example, a good balance between errors and cost was achieved using a subaperture size of 5 elements, focus at 40 mm range, and a delay quantization step corresponding to a phase of π/4.

Image revivals in multi-mode optical fibers with periodic multiple sub-apertures

We report experiments on a multi-mode fiber-based device that reimages the input pattern after specific propagation distances. The reimaging has two propagation length scales related to the Talbot self-imaging in a periodic grating and image revival effects. We use a beam propagation method to simulate diffraction and refraction of light in the optical fiber. The details of the fiber preparation and optical experiments are described. We study the optical imaging properties using a close-packed array of sub-apertures placed at regular positions on a triangular lattice. We numerically analyze the propagation, diffraction and coupling characteristics of the beam oscillating inside the fiber. Our simulations identify the optimal reimaging length of the multi-mode (MM) fiber to get high fidelity image revival. Experiments are performed to validate the simulation results.

Hybrid spiral plasmonic lens: towards an efficient miniature circular polarization analyzer

A hybrid spiral plasmonic lens that consists of alternating spiral slot and spiral triangular sub-aperture array can differentiate circular polarization of different handedness and enable a miniature circular polarization analyzer design with high efficiency. The improved performance compared to pure spiral slot lens comes from the fact that the hybrid lens is capable of focusing both the radial and the azimuthal polarization components of a circular polarization, doubling the coupling efficiency. In this paper, the spin-dependent plasmonic focusing properties of a spatially arranged triangular sub-aperture array and a hybrid spiral plasmonic lens are demonstrated using a collection mode near field scanning optical microscope. The coupling efficiency could be further improved through optimizing the geometry of the hybrid lens.

Fresnel phasing of segmented mirror telescopes

Shack-Hartmann (S-H) phasing of segmented telescopes is based upon a physical optics generalization of the geometrical optics Shack-Hartmann test, in which each S-H lenslet straddles an intersegment edge. For the extremely large segmented telescopes currently in the design stages, one is led naturally to very large pupil demagnifications for the S-H phasing cameras. This in turn implies rather small Fresnel numbers F for the lenslets; the nominal design for the Thirty Meter Telescope calls for F=0.6. For such small Fresnel numbers, it may be possible to eliminate the lenslets entirely, replacing them with a simple mask containing a sparse array of clear subapertures and thereby also eliminating a number of manufacturing problems and experimental complications associated with lenslets. We present laboratory results that demonstrate the validity of this approach.

Whole-Surface Characterization of Progressive Addition Lenses

To characterize and compare progressive addition lens (PAL) surfaces in terms of the Zernike polynomials. We examine the spherocylindrical characteristics of PALs by performing a physical surface height measurement of three lenses. Surface shape is described as the sum of Zernike polynomials. These data were used to analyze and compare each surface, using the second derivatives of surface height. We compare PALs to each other and to a prototypical progressive surface, i.e., the Alvarez lens surface. The high order freeform surfaces of PALs produce a gradient of spherical power across the lens aperture and also a profile of astigmatic power with high order aberration. Each lens is similar to the Alvarez surface, but each has characteristics that differentiate it from the others. Each high order Zernike term contributes sphere and/or astigmatism, and aberration, to the surface. This approach to PAL analysis, in which the entire lens surface is defined and expressed as the sum of Zernike polynomial terms, is different from most previous approaches in which a lens is described as an array of subapertures. It is potentially an efficient and useful method for defining and designing PAL surfaces.

Performance test and closed-loop correction experiment of a 20-element bimorph deformable mirror

Performances of a 20-element bimorph deformable mirror are measured and a closed-loop adaptive optics system, which consists of the bimorph mirror and a Shack-Hartmann wave-front sensor with 13×13 sub-aperture array, is established to test the mirror’s capability of correcting static aberrations. Experimental results show that, the mean displacement response to voltage of discrete actuators is 0.57 μm/100 V and that of defocus electrode is 1.09 μm/100 V. The hysteresis of actuator is limited to ten percents of the maximal displacement. With the ambient temperature increasing from 13℃ to 25℃, the P-V of defocus of the mirror surface is enlarged by 1.37 μm and the P-V of other distortions, such as astigmatism, coma and spherical aberration, is enlarged by 0.15 μm. The frequency response of the bimorph mirror is about 4 kHz. When the closed-loop control is on, intensity distributions at the image plane of distorted wave-fronts are effectively improved. The Strehl ratios increase from 0.02 to 0.95, and 0.03 to 0.54， respectively.

Imaging with an array of adaptive subapertures

An imaging system composed of an array of adaptive optics subapertures referred to as a conformal imaging system is considered. A conformal image of an object viewed through atmospheric turbulence is obtained using the following sequential steps: adaptive compensation of phase distortions through optimization of image quality metrics at each subaperture, measurements of the phase and intensity distributions corresponding to the compensated subaperture images, digital combining and processing of the obtained data, computation of a conformal image using arbitrary phase shifts between subapertures, and correction of these phase shifts through conformal image quality optimization using the stochastic parallel gradient descent algorithm. Numerical simulation results of a dual-star conformal image through atmospheric turbulence are presented.

Ground‐Layer Wave Front Reconstruction from Multiple Natural Guide Stars

Observational tests of ground layer wavefront recovery have been made in open loop using a constellation of four natural guide stars at the 1.55 m Kuiper telescope in Arizona. Such tests explore the effectiveness of wide-field seeing improvement by correction of low-lying atmospheric turbulence with ground-layer adaptive optics (GLAO). The wavefronts from the four stars were measured simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x 5 array of square subapertures across the pupil of the telescope, allowing for wavefront reconstruction up to the fifth radial Zernike order. We find that the wavefront aberration in each star can be roughly halved by subtracting the average of the wavefronts from the other three stars. Wavefront correction on this basis leads to a reduction in width of the seeing-limited stellar image by up to a factor of 3, with image sharpening effective from the visible to near infrared wavelengths over a field of at least 2 arc minutes. We conclude that GLAO correction will be a valuable tool that can increase resolution and spectrographic throughput across a broad range of seeing-limited observations.

Advances in transducers and techniques for diagnostic ultrasound

Until recently, diagnostic imaging systems have remained stable with familiar modesto be measured. Emerging technologies are changing the measurement landscaperapidly. New developments are reviewed as well as those in research to examine thepossible measurement challenges of the future. Improved focusing and steering areavailable with 1.5 D, multiple subaperture arrays, fully populated and sparse 2Darrays, and CMUT arrays. Encoded excitation pulses provide unique consideration.High frequency and special purpose arrays stretch the present limits of measurementbandwidth and resolution. Special auxiliary pulses and pulse sequences are beingproposed for controlling, manipulating and destroying contrast agents and speciallyformulated therapeutic contrast agents. Improved harmonic imaging can involvemultiple phased pulses as well as types of encoding. Active tissue characterizationmethods may include a means for tissue deformation as well as aberration correction.Imaging systems are undergoing a dramatic revolution in terms of their architecture,complexity, miniaturization and the role of software in image formation. Thesedevelopments will be examined in terms of field distribution, pressure levels, pulsesequencing, waveforms and measurement

Space-time processing for multichannel synthetic aperture radar

Synthetic aperture radar (SAR) provides high-resolution images of a non-moving ground scene, but fails to indicate the presence and position of moving objects. As in airborne MTI (moving-target indication) systems the solution to this problem is to use an array of antennas or subapertures and several receiving channels ('MSAR', or multichannel SAR), and to apply multichannel clutter suppression. One of the most efficient methods is adaptive space-time processing (STAP), which can be simplified to frequency-dependent spatial processing in the Doppler domain. Some of these techniques applied to SAR are reviewed and illustrated with data gathered by the German experimental multichannel SAR system 'AER-II'.

Image characteristics in applications utilizing dilute subaperture arrays

When a large aperture is synthesized with an array of smaller subapertures for high-resolution imaging applications, it is important not only to arrange the subapertures to achieve minimal spatial frequency redundancy but also to choose the size of the subapertures (i.e., the dilution ratio) necessary to achieve the best possible image quality. Spurious or ghost images often occur even for nonredundant dilute subaperture arrays. We show that array configurations producing a uniform modulation transfer function will not exhibit these undesirable ghost images. A prescription that is unique and original (to the best of our knowledge) is then presented for constructing both one-dimensional and two-dimensional configurations of dilute subaperture arrays that results in a uniform spatial frequency response with an arbitrarily high spatial resolution for reciprocal path-imaging applications.

Sparse array configurations yielding uniform MTFs in reciprocal path imaging applications

A method is presented for constructing both one-dimensional and two-dimensional configurations of sparse subaperture arrays which result in uniform spatial frequency response with arbitrarily high spatial resolution for reciprocal path imaging applications (i.e., imaging laser radar applications).

Reconstruction of turbulence-degraded images using nonredundant aperture arrays

A technique is described which allows the removal of seeing distortions from a single frame of speckle-type imagery, provided that this frame is obtained using an aperture consisting of a nonredundant array of subapertures, each smaller than the seeing correlation length. Although performed a posteriori, the method is related to those already proposed for use with active optical systems. Computer simulations are decribed which verify the basic features of this technique. The simulations indicate that reconstructed images of diffraction-limited quality should be obtainable for starlike objects as dim as eighth magnitude. For more extended objects, the limiting magnitude depends somewhat on the object structure. The technique described is immediately applicable to any large telescope, and because the processing is done after the fact, a frame containing many isoplanatic patches may be processed piecewise, allowing the reconstruction of large areas.